Family histories studies showed a familial aggregation of Alzheimer's disease among first-degree relatives of Alzheimer's disease probands compared with controls. A case report shows that dementia in Down syndrome may occur without mental retardation. DNA repair studies show a G2 radiation repair deficiency in Down syndrome. Buffy coats from blood of first-degree relatives with Alzheimer's disease were injected into hamsters to test transmissibility in Alzheimer's disease. In animals surviving up t 301 days after inoculation, no evidence of brain disease was present, arguing against transmissibility.